Sublimation dye media generally comprise a support having coated thereon in one or more layers a dye donor layer comprising a thermally mobile sublimation dye dispersed or dissolved in a polymeric binder, and are becoming very important for producing colour images, especially with color gradation. In particular, sublimation dye media are being combined with thermal printheads to produce digital colour hardcopy by sequential deposition of yellow, magenta and cyan dyes. To obtain the copy, a cyan, magenta or yellow dye-transfer-medium is placed in face-to-face contact with an image (dye-receiving) receptor. The assembly of media and receptor is then inserted between the thermal print head and a platen roller. A line-type thermal print head is used to apply heat from the back, i.e., the uncoated surface of the support of the media, to cause dye transfer in the heated areas. The thermal print head typically comprises a plurality of heating elements and is heated up sequentially in response to the signals transmitted to the print head. The process is then repeated for the remaining two colours to obtain a full colour hard copy of the original image. Further details of this process and apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271 entitled Apparatus and Method for Controlling a Thermal Printer Apparatus. Such a process, however, requires the use of a thin substrate for the dye-transfer-medium in order to maximise the operation of the thermal printhead which prints pixels in a matter of milliseconds. Thus, highly sensitive media on thin substrates, i.e., 8 .mu.m or less, need to be designed, which do not suffer from shelf life constraints. Less sensitive media, on thicker, more easily coatable substrates, i.e., 10 .mu.m or greater, are found to increase wear and tear of the printhead.
It is also known to incorporate infrared absorbing materials in the dye-donor layer to allow imaging via exposure to an infrared laser, such as a laser diode. The infrared absorbing material generates heat in the exposed areas, causing dye transfer in those areas. British Patent Publication No. 2083726 discloses the use of carbon black for this purpose, while U.S. Pat. Nos. 4,942,141, 4,948,7768, 4,950,639, 4,950,640 and 4,952,552 describe specific classes of infrared dyes for use in this way.
Dye-transfer-media for thermal dye transfer printing comprising a dye donor layer coated directly on a support are found to experience loss of dye by uncontrolled, non-directionalised diffusion into the support during both storage and the actual transfer process. The support often softens during heating and has the inherent property to act as a receiver for the dye. Dye which is lost by this `wrong way` diffusion results in less dye being transferred to the image receptor. Since the background density in a thermal-dye-transfer system is essentially constant, any increase in density of the transferred dye in image areas results in improved discrimination which is highly desirable.
U.S. Pat. Nos. 4,716,144 and 4,700,208 discloses dye donor elements for thermal dye transfer which comprise a support having on one side thereof a dye donor layer and on the opposite side thereof a slipping layer comprising a lubricating material, a hydrophilic dye-barrier layer located between the dye donor layer and the support, and a subbing layer located between the dye-barrier layer and the support. Any thermally transferable dye(s) may be used in the dye donor elements but sublimable dyes are preferred. The hydrophilic barrier layer is said to prevent bidirectional transfer of dye into the subbing layer/support with the result that the density of the transferred dye is increased. Preferred hydrophilic materials are said to include poly(acrylic acid), cellulose mono-acetate and poly(vinyl alcohol).
Self-supporting ink formulations are known and disclosed, for example, in U.S. Pat. Nos. 4,609,928 and 4,103,066 and in European Patent Publication No. 120230. All the examples relate to mass transfer inks, from which transfer of both colourant and binder to the receptor occurs on heating. Sublimation media, on the other hand, transfer only the colourant to the receptor and thus can yield graded (i.e., grey scale) response to varying thermal energy. The mass transfer process is only bi-level generally.
U.S. Pat. No. 4,857,503 discloses the concept of a dye-transfer-medium comprising a single, self-supporting layer of a polymeric binder having dispersed or dissolved therein a thermally transferable dye. However, there is no actual reduction to practice as it is recognised that such media would transfer much lower amounts of dye, when compared with multilayer dye-transfer-sheets comprising dye and binder coated onto a carrier substrate. This results from the high ratio of binder:dye required to provide the media with sufficient structural integrity. Furthermore, when such media are used in thermal printers, the tendency of the dye to transfer bilaterally, that is, onto the print head as well as the receptor substrate, necessitates more frequent (and often laborious) cleaning and in some cases a reduction in the print head's life expectancy.
The present invention seeks to provide alternative dye-transfer-media.